/Engine/Source/Runtime/Engine/Public/AnimationRuntime.h
Get Key Indices (start/end with alpha from start) with input parameter Time, NumKeys from % from StartKeyIndex, meaning (CurrentKeyIndex(float)-StartKeyIndex)/(EndKeyIndex-StartKeyIndex) by this Start-End, it will be between 0-(NumKeys-1), not number of Pos/Rot key tracks The FramesPerSecond parameter must be the sample rate of the animation data, for example 30. If the FramesPerSecond parameter is set to 0 or negative, it will automatically calculate the FramesPerSecond based on the sequence length and number of frames. The reason why you can provide a FramesPerSecond value is because this can be slightly more accurate than calculating it, in case super high precision is needed.
static void GetKeyIndicesFromTime ( int32 & OutKeyIndex1, int32 & OutKeyIndex2, float & OutAlpha, const double Time, const int32 NumKeys, const double SequenceLength, double FramesPerSecond )